Problem: $-\dfrac{3}{12} + \dfrac{9}{5} = {?}$
$ = - {\dfrac{3 \times 5}{12 \times 5}} + {\dfrac{9 \times 12}{5 \times 12}} $ $ = - {\dfrac{15}{60}} + {\dfrac{108}{60}} $ $ = - \dfrac{{15} + {108}}{60} $ $ = \dfrac{93}{60}$